Electrical boxes (also referred to as “junction boxes”) are used to house and protect electrical devices such as electrical outlets, switches and associated wiring when the devices are installed in a building. Typically, an electrical device is installed either by attaching the junction box containing the device directly to framing within the structure, or by attaching the box to a mounting device that is fastened to such framing. One such box mounting device is described in U.S. Pat. No. 5,209,444. The device comprises a mounting bar having fastening flaps at opposite ends of the bar for attachment to spaced apart wall studs. When the flaps are attached to the studs, the bar extends generally horizontally between the studs. The bar has a generally channel-shaped cross-section, and a plurality of box mounting openings are spaced along the length of the bar for receiving fasteners to attach an electrical box to the bar at a selected location. Another box mounting device is described in co-assigned published application Ser. No. 11/093,145, filed Mar. 29, 2005 and published on Oct. 26, 2006 as U.S. Pub. No. US2006/0237601. This publication discloses an electrical box mounting bracket which is adjustable along a mounting bar to a selected position corresponding to selected fastener openings in the bar.